Such reactors are known for both aerobic and anaerobic treatment of water. An example of an embodiment for anaerobic treatment is described in U.S. Pat. No. 4,609,460.
A driving force for the downward flow of recirculation water and sludge is obtained by producing a gas lift effect in such a treatment (in which case the gas-containing liquid outside the downer is lighter than the non-gas-containing liquid in the downer).